As is known, many food products, such as fruit juice, UHT milk, wine, tomato sauce, etc., are sold in packages made of sterilized sheet packaging material.
A typical example of this type of package is the parallelepiped-shaped package for liquid or pourable food products known as Tetra Brik Aseptic (registered trademark), which is made by folding and sealing laminated strip packaging material.
The packaging material has a multilayer structure substantially comprising a base layer for stiffness and strength, which may be defined by a layer of fibrous material, e.g. paper; and a number of layers of heat-seal plastic material, e.g. polyethylene film, covering both sides of the base layer.
In the case of aseptic packages for long-storage products, such as UHT milk, the packaging material also comprises a layer of gas- and light-barrier material, e.g. aluminium foil or ethyl vinyl alcohol (EVOH) film, which is superimposed on a layer of heat-seal plastic material, and is in turn covered with another layer of heat-seal plastic material forming the inner face of the package eventually contacting the food product.
As is known, packages of this sort are produced on fully automatic packaging machines, on which a continuous tube is formed from the web-fed packaging material. The web of packaging material is unwound off a reel and fed through a sterilizing unit, where it is sterilized, e.g. by immersion in a chamber of liquid sterilizing agent, such as a concentrated solution of hydrogen peroxide and water.
The web is then fed into an aseptic chamber where the sterilizing agent is evaporated by heating. The web is then folded into a cylinder and sealed longitudinally to form in known manner a continuous vertical tube, which in effect forms an extension of the aseptic chamber. The tube of packaging material is filled continuously with the pourable food product and then fed to a form-and-seal unit, where it is gripped between pairs of jaws which seal the tube transversely to form pillow packs. The pillow packs are then separated from one another by cutting the seal joining each two adjacent packs, and are conveyed to a final folding station where they are folded mechanically into the finished shape.
More specifically, the sterilizing unit comprises a chamber containing the sterilizing agent, and into which the web is fed continuously. The sterilizing chamber conveniently comprises two parallel vertical branches connected at the bottom to define a U-shaped path long enough with respect to the travelling speed of the web to allow enough time to treat the packaging material.
For effective, relatively fast treatment, e.g. in about 7 seconds, to reduce the size of the sterilizing chamber, the sterilizing agent must be maintained at a high temperature, e.g. of around 73° C. In known sterilizing units, this can be done, for example, by fitting electric heaters to the walls of the vertical branches of the sterilizing chamber.
Being covered with a layer of heat-seal plastic material, normally polyethylene, the faces of the web of packaging material are completely impermeable to the sterilizing agent. Along the edges of the web, however, the layer of fibrous material is exposed, and tends to soak up the sterilizing agent. This is known in the trade as “edge wicking”, and remains within acceptable limits providing the web is only kept for a short time inside the sterilizing chamber, as is the case during normal operation of the machine.
If for any reason the machine is stopped, however, the sterilizing chamber must be emptied immediately. Otherwise, the edges of the layer of fibrous material soak up the sterilizing agent, and edge wicking of a few millimetres in width inevitably impairs subsequent longitudinal sealing of the web to form the tube of packaging material as described above.
In other words, in the event the machine is stopped, the sterilizing agent is drained rapidly into a normally double-walled hold tank. The inner walls define an inner shell of the tank containing the sterilizing agent; and the outer walls form an outer shell of the tank defining, with the inner shell, a normally air-filled gap which provides for thermally insulating the sterilizing agent.
In known machines, in the event of a short stoppage, normally of no more than 15-20 minutes, and particularly when starting up the machine again, edge wicking tends to occur anyway, despite emptying the sterilizing chamber.
Careful study of the phenomenon has identified several causes, foremost of which are:                the porosity of the fibrous material, which, however, can only be reduced so far for paper manufacturing cost reasons;        hydrostatic pressure, which is also difficult to reduce, on account of the height of the U-shaped sterilizing chamber depending on the necessary processing time, and only being reducible by altering the architecture of the sterilizing unit, thus complicating the system as a whole; and        the temperature of the sterilizing chamber during the stoppage, and of the sterilizing agent when fed back into the chamber.        
As regards the latter, in particular, a difference of even only a few degrees between the temperature of the chamber during the stoppage and the temperature of the sterilizing agent fed back into the chamber has been found to produce severe edge wicking. In conventional machines, this difference in temperature is caused by the tendency of the emptied sterilizing chamber to increase in temperature, on account of the inevitable delay in response of the thermostatic control to the reduction in heat absorption caused by emptying the chamber: the temperature inside the chamber is therefore normally around at least 80° C. As a result, the residual sterilizing agent on the walls of the chamber and in the packaging material tends to evaporate, thus producing a saturated-vapour condition of the chamber, so that the pores of the layer of fibrous material contain a saturated air/vapour mixture.
When liquid sterilizing agent is fed into the chamber, at a temperature inevitably lower than that inside the sterilizing chamber, the temperature of the web, and therefore of the air/vapour mixture in the pores, is reduced. This reduction has a practically negligible effect on the air, which undergoes a contraction in volume of only a few percent, but has a very serious effect on the vapour, which recondenses and so assumes a much smaller volume in the liquid state. This drastic reduction in volume has the effect of “sucking” the sterilizing agent into the pores of the fibrous material layer, which is the major cause of edge wicking.
By way of a solution to the problem, sterilizing units have been devised, in which the sterilizing agent is heated before being fed into the sterilizing chamber.
In the case of prolonged stoppages, as, for example, at the end of the package production cycle, simply draining the sterilizing agent into the hold tank is sufficient to prevent edge wicking, even when the packaging machine is started up again. During a prolonged stoppage, in fact, the residual liquid inside the sterilizing chamber evaporates, and relative humidity inside the chamber is reduced. When the packaging machine is started up again after an end-of-cycle shutdown, the sterilizing chamber and the packaging material inside it are perfectly dry.
In known sterilizing units of the above type, the sterilizing chamber and other sterile parts of the packaging machine are ventilated at the end of each production cycle to remove any residual sterilizing agent, by blowing in air, which is then sprayed with water to eliminate the residual sterilizing agent.
Packaging machines of the above type are used widely and satisfactorily in a wide range of food industries; and performance of the sterilizing unit, in particular, is such as to ensure a wide margin of safety as regards regulations governing aseptic packages and the permitted amount of residual sterilizing agent.
A need is felt within the industry, however, for further improvement, particularly as regards the average “life” of the sterilizing agent, i.e. the average length of time the sterilizing agent remains effective.